1. Field of the Invention
The present invention relates to a variable geometry turbine and a turbocharger having the same.
This application is based on Japanese Patent Application No. 2008-051044, the content of which is incorporated herein by reference.
2. Description of Related Art
A conventionally known turbine provided for a variable geometry turbocharger is, for example, the technology disclosed in Japanese Unexamined Patent Application, Publication No. HEI-10-8977. The turbine includes communicating parts (hereinafter, referred to as “throats”) that radially divide a scroll portion into an inner scroll part and an outer scroll part and communicate between the inner scroll part and the outer scroll part, and a switch valve that switches the flow of fluid between a flow into the inner scroll part and a flow into both the inner and outer scroll parts. The flow path is switched by the switch valve depending on the flow rate of the fluid.
According to the above-mentioned technology, when the flow rate of the fluid is low, the switch valve is operated to make the fluid flow only into the inner scroll part to apply a rotary torque to a turbine rotor, making the turbine rotor rotate effectively. Further, when the flow rate of the fluid is high, the switch valve is operated to make the fluid also flow into the outer scroll part to make the mass flow uniform and apply a small rotary torque to the turbine rotor, preventing the turbine rotor from rotating more than necessary.
However, in the above-mentioned technology, when the flow rate of the fluid is high, the flow rate of fluid flowing from the outer scroll part into the inner scroll part via the throats changes depending on the positions of the throats. Specifically, as shown in FIG. 14, the flow rate of the fluid flowing from the outer scroll part into the inner scroll part via the throats increases in the downstream direction of the scroll portion. Fixed-vane throat numbers shown in FIG. 14 are sequentially assigned to the throats from the upstream side of the scroll portion. Further, when the flow rate of the fluid is low, the fluid flows from the inner scroll part into the outer scroll part via the throats. As a result, as shown in FIG. 15, outward flows via the throats occur upstream of the scroll portion and inward flows via the throats occur downstream of the scroll portion. Variations in the flow rate at the throats, shown in FIGS. 14 and 15, reduce the performance of the turbine.